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Effect of Feed Rate on GRG Recovery and Concentrate Grade (4 replies)

Carl Jenkins
9 months ago
Carl Jenkins 9 months ago

Anyone experience any significant correlation between feed rate and grade/recovery in GRG concentration test work?If we run 50kg batch, 20 kg batch, 2 kg batch with similar feed rate and other parameters, anyone experience any significant difference in GRG grade and recovery?

Maya Rothman
9 months ago
Maya Rothman 9 months ago

My first guess would be start up and shut down issues. In any operation the times when you have the most variation is in starting and stopping the run. Even with laboratory equipment, it takes time for the process to stabilize. If you are running three different size test runs with "similar feed rate and other parameters", one of the parameters must be different, length of run. To run a 2 kg batch and a 20 kg batch "similar feed rate and other parameters", the run length would need to be 10 times as long. And the 50 kg batch another 2.5 times as long.Start by taking incremental samples, and keep them separate. Analyze them individually and see if there is a run time variation.

9 months ago
Oberfuhrer 9 months ago

As the volume of concentrate is imposed by the equipment (laboratory Knelson for example), the main impact of the batch size will be the concentration! For 2 kg you will have a low grade concentrate, for 20 kg a higher grade. But the grade cannot increase indefinitely and if the batch is too large, you loss gold in tailings. It is why he was proposing a "standardised" protocol for comparison between various ores.

Another difference is the sample representativeness. 50 kg batch is more representative than 20 kg. If the sample to be characterized is large, it can be necessary to split it in many batches for concentration.

Zander Barcalow
9 months ago
Zander Barcalow 9 months ago

A paper was presented at the GeoMet 2013 Conference on this very topic : "Variability of Gravity Recoverable Gold Values in Relation to Test Sample Size" by S.C.Dominy, pages 293-301. His conclusions were as follows:

"General wisdom states that plant GRG values are usually 33 per cent to 66 per cent of those defined by testing (Clark, 2005; Koppalkar et al, 2011). In this case, SGRG consistently understate that achieved in the plant. This can potentially be explained a number of ways:

•This is a highly heterogeneous orebody, where only the background low-medium grade mineralisation was assessed.

•The small samples (≤50 kg) have minimal chance of containing coarse gold, representing between 0.03 per cent and 0.3 per cent of the development round. The fine gold component is relatively more disseminated and has a high probability of occurring in all samples.

•The target grind product of the plant grinding mill was P80-500 μm. However, the SGRG mill product was generally finer (P80 -250 μm), potentially resulting in some over ground gold reporting to the non-GRG component.

•Loss of flat (oblate) gold particles >0.5 mm into the tails during SGRG testing was occasionally observed. This gold was generally captured in the plant via the gold trap.

•The discrepancy between the 250 kg SGRG and BS KCGRG recoveries may in part be explained by the use of two in series concentrators. Gold lost in the tails of KC1 has a chance of being recovered in KC2, whereas there is no second chance during the SGRG test. As noted previously, some gold may be lost from the SGRG test.

•The discrepancy between the BS KC-GRG and BS All-GRG is explained by the loss of >0.5 mm generally oblate gold particles, which were recovered in the trap prior to the Knelson circuit. Routine processing indicated that between ten per cent and 40 per cent of gold reports to the gold trap, particularly when grades exceed 15 g/t Au.

General conclusions related to GRG evaluation:

•The SGRG approach is a useful one, but the accepted sample mass of 20 kg may be too small for heterogeneous low-medium grade ores.

•Consideration of gold particle size distribution is important, since small samples may only be resolving a part of the distribution. The coarse to very coarse (>500 μm) fraction is potentially the most material to GRG results. Early characterisation is required to understand the nature of the mineralisation.

•Representability must be driven by appropriate mass to improve precision and good sampling protocols to reduce bias. A preliminary estimate of sample mass can be defined by the application of Poisson statistics following ore characterisation.

•Caution is given as to the use of composite samples without understanding their spatial and geological-metallurgical context and the effect of compositing. However, in the heterogeneous environment well-chosen composites may be the best way to represent the entire gold particle size distribution. Consideration of the number of samples collected is also important.

•Technical teams should consider bulk sample/pilot testing programs to evaluate grade and metallurgy as part of feasibility studies (Dominy, Platten and Xie, 2008). This study recommends caution during the interpretation of GRG results in heterogeneous gold mineralisation. It is important that the likely representability of the sample be placed in the context of grade, gold particle size distribution, gold deportment and spatial variability. The consequences of not undertaking proper ore characterisation early in the mine development chain relates to unrepresentative samples thus ill-informed plant design, leading potentially to negative economic consequences."

Alan Carter
9 months ago
Alan Carter 9 months ago

Totally agree with all the above. The nugget effect calls for a larger sample size. Start-up and shutdown effects will bias results so the longer you are at steady state, the better.

You need to think of all the variables and plan the sample size to minimise the impacts of all the process disturbances. Also think about the size distribution of the heavies you want to capture. Setting up a Knelson to capture coarse gold will miss gold in fines; and vice versa. Think about the likely plant flowsheet and try to mimic that as much as possible or you will get data that will not be reproducible in the plant. If your size distribution is too narrow or broad you will never be able to duplicate it in a plant!

Talk to the experts and remember that GRG comes in all forms some recoverable and some not. I was at one mine site that had 40% GRG according to the vendor testwork, but after taking into account the refractory gold content there was a 1% recoverable gold content in the GRG unless you used BIOX or autoclaving!

So what to do with the con? Gekko ILR or Acacia reactor or oxidise or ultrafine grind followed by intensive leach? Look at it holistically and you will do alright. Knowing you can recover gold is only the beginning of your journey enjoy it and keep your eyes wide open.

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